If you have an WNDR3800 with stock firmware, and the version is greater than 1.0.0.16, then you will first need to downgrade to firmware no greater than 1.0.0.16 (from here). Netgear put a check in firmware greater than 1.0.0.16 to allow only Netgear firmware from the Web GUI. It hasn't been confirmed if you can flash DD_WRT using TFTP with stock firmware greater than 1.0.0.16.

You will need these files for the flashing procedure.

If you have a North American model, you will need this for the initial flash.

Should you want to use DHCP ip assignment, remember to un-set the static ip that you set in the above instructions.

This unit was supported as of build 18687.

For updating to a newer dd-wrt build, you should be able to use the wndr3800-webflash.bin mentioned above. Make sure to reset to defaults with a hard reset as listed above and download the appropriate dd-wrt build that you are wanting to update to. New builds can be found within the dd-wrt file server: